Xerographic developing apparatus



Aug. 21, 1956 R. cs. VYVERBERG XEROGRAPHIC DEVELOPING APPARATUS 2 Sheets-Sheet 1 Filed Oct. 26, 1955 INVENTOR. ROBERT G. VYVERBEFQG BY 1, 1956 R. G. VYVERBERG 2,759,450

XEROGRAPHIC DEVELOPING APPARATUS Filed Oct. 26, 1955 2 Sheets-Sheet 2 INVENTOR. ROBERT G. \IYVERBERG BY vg ydgwzw ATTORNEY United States Patent XERQGRAPHIC DEVELOPING APPARATUS Robert G. Vyverherg, Pittsford, N. Y., assignor to The Haloid Company, Rochester, N. Y., a corporation of New York Application October as, 1955, Serial No. 542,907

12 Claims. (Cl. 118309) This invention relates to the field of xerography and, particularly, to improvements in xerographic developing apparatus,

More specifically, the invention relates to an improved type of closed-cycle, xerographic developing unit that is particularly adapted for use in xeroradiography wherein, in order to develop the electrostatic latent image of a radiogra'ph formed on a xerographic plate, it is required to generate a dense cloud of oppositely charged powder particles and to direct such cloud against the surface of the plate, whereby the powder particles are selectively attracted to the charged areas of the plate to form a visible powder particle image of the electrostatic latent image.

In the process of xerography, for example, as disclosed in Carlson Patent 2,297,691, issued October 6, 1942, a xerographic plate comprising a photoconductive insulating material on a conductive backing is given a uniform electric charge over its surface and is then exposed to the subject matter to be reproduced, usually by conventional projection techniques. This exposure discharges the plate areas in accordance with the light intensity which reaches them, and thereby creates an electrostatic latent image on or in the plate coating. Development of the image is effected with an electrostatically charged, finely-divided material, such as an electroscopic powder, which is brought into surface contact with the coating and is held thereon electrostatically in a pattern corresponding to the electro static latent image. Thereafter, the developed xerographic image is usually transferred to a support material to which it may be fixed by any suitable means. In the application of the art of xerography to the field of radiography, as described in a paper entitled, Industrial Xeror'adiography in 1955 by Robert G. Vyverberg, Harold E. Clark, and John H. Dessauer, and appearing in the May- June 1955, issue of Non-Destructive Testing, published by The Society for Non-Destructive Testing, of Evaston, Illinois, the basic methods are essentially the same as those described above except that the developing techniques differ from those commonly employed in the reproduction of line copy or continuous tone subject matter.

These differences come about partly because of basic difierences in the type of electrostatic latent image formed in each case, and partly because of the different end re-- sult that is required in each application. For example, in the reproduction of line copy, the electrostatic latent image formed on the xerographic plate constitutes a charge pattern of substantially constant voltage in the unexposed areas of the plate, whereas the exposed areas of the plate are at substantially zero voltage. In the reproduction of continuous tone copy, the electrostatic latent image remaining on a xerographic plate after exposure constitutes a charge pattern that varies substantially pro: portionally to the tone densities of the various areas of the copy to be reproduced. In the formation of xeroradiographs, the situation is somewhat the same as that in continuous tone work, namely, that the electrostatic latent image formed on the xerographic plate has a varying charge in accordance with differences in' density or difierences in material within the subject matter that is being radiographed. However, in addition, it is also desired that the xeroradiograph have sulficient contrast sensitivity to accentuate discontinuities on or in the subject matter.

In order to attain the desired end results in each type of work, different developing materials and developing techniques are required in each instance. For example, in the development of the electrostatic latent image of line copy, the developing material comprises a pigmented powdered resin that is combined with coated glass bead carriers and is thereby triboelectrically charged to a polarity opposite to that of the electrostatic image to be developed. Subsequently, the combined developing material is cascaded over the electrostatic latent image whereby the resin particles are caused to adhere electrostatically to the charged portions of the image. Since none of the resin particles adhere to the exposed or uncharged areas of the xerographic plate, the resulting developing material pattern corresponds to the line copy to be reproduced.

In continuous tone applications of xerography, the problem is more complicated because of the essentially solid area coverage that is required. In these cases, areas of higher charge not only attract more developing powder than those of lesser charge but, in addition, create fringing electrostatic fields that tend to attract developing powder to the edge of the area of higher charge and leave the edge of the area of lesser charge substantially devoid of powder. In order to eliminate the effect of this phenomenon, continuous tone work is preferably developed with the use of a development electrode of the type disclosed in co-pending application S. N. 185,387, filed in the name of Lewis E. Walkup on September 18, 1950. Briefly, the development electrode comprises a metallic plate that is arranged in spaced relation to the xerographic plate to be developed and, in its general application, is maintained at ground potential with reference to the electrostatic latent image formed on the xerographic plate. This arrangement completely neutralizes the effect of the fringing fields between areas of differing charge whereby, when a developing agent is electrostatically charged oppositely to the electrostatic latent image and is brought into surface contact therewith, the developing material particles adhere to the image surface in differing densities depending solely upon the amount of charge in any given portion of the latent image.

In the development of xeroradiographs, the unequal attraction of developing powder to the latent image caused by the phenomenon of fringing fields is quite advantageous to a point since, as mentioned above, it tends to emphasize small discontinuities. However, at sharp discontinuities in the charged pattern on the plate, such as would be formed from voids or steps in the radiographic subject, excessive fringing fields are sometimes created which tend to obscure discontinuities that should be observable in the finished xeroradiograph.

Attempts to modify the effect of these fringing fields by the use of a development electrode of the type mentioned above, have in general been unsuccessful because xeroradiographs are usually formed on a plate that is too large conveniently to permit uniform distribution of a developing material when such a development electrode is employed. However, it has been found that proper development can be effected by supporting a xerographic plate with its electrostatic latent image facing downwardly in a large box, and by spraying a cloud of oppositely charged, finely divided developing powder into the box.

By this means, it is believed that the charged powder cloud forms a virtual electrode in front of the plate andat some distance therefrom. This virtual electrode creates arelatively weak field between it and the charged areas of the plate that is effective to modify the action of the fringing fields sufiiciently so that the powder particles are attracted to the several plate areas substantially in proportion to the charge on such areas. However, sufficient fringing fields remain at sharp discontinuities to attract some additional powder to the edge of the area of higher charge while leaving the edge of the area of lesser charge somewhat underdeveloped but, at the same time, not creating such an unbalance of powder distribution as to obscure lesser discontinuities that may be present in the vicinity. It has been found that the efficiency of this type of development is a function of the density of the powder cloud and of the amount and homogeneity of the charge on the cloud. That is, the denser the cloud and the higher the charge, the virtual electrode formed thereby is apparently closer to the plate and is more effective in producing the desired results.

A developing apparatus for producing a powder cloud of the type described above is disclosed in co-pending application S. N. 436,482 filed in the name of Herbert E. Crumrine, et al., on June 14, 1954. In this device a xerographic plate is supported over an opening in a developing box so that the charged image on the plate faces the interior of the box. Powdered developing material is then forced into the box by means of a compressed air driven, powder spray gun that also serves to apply the requisite charge to the powder, and the exhaust powder is drawn from the box through a filtering device. Although devices of this type are in extensive use they are quite bulky and are expensive to manufacture. In addition, these systems require relatively long operating periods in order to complete the development of xeroradiographs. Finally, such systems are rather diificult to maintain and are wasteful of developing material.

The principal \object of the present invention is to provide an improved xeroradiographic developing apparatus. A further object is to provide a high speed, closed cycle, xerographic developing apparatus for forming a very dense and highly charged cloud of developing materials that can conveniently be applied to an electrostatic latent image on a xerographic plate. A further object is to provide a regenerative xerographic developing system to eliminate waste of xerographic developing material. A further object is to provide an etficient, compact, and low cost developing apparatus that is simple to operate and inexpensive to maintain.

These and other objects of the invention are attained by means of a developing powder agitator and a developing powder impeller that are coaxially mounted in spaced relation on a vertical shaft driven by a high speed motor and are enclosed at the lower end of a container or powder cloud chamber having upwardly divergent side walls that terminate in a substantially horizontal plane and are enclosed at their top by a cover portion having a mounting apparatus for supporting a xeroradiographic plate with its photoconductive surface exposed to the interior of the cloud chamber. By this arrangement there is provided a closed cycle, xerographic developing apparatus that is simple to operate and maintain, that eliminates the need for the air compressor, spray gun and filter mechanism employed in previous systems of this type, and that functions regeneratively to make complete and eflicient use of all developing material inserted in the system.

The invention is disclosed in the appended drawings, in which:

Fig. l is an isometric view of a xerographic developing apparatus constructed in accordance with the invention;

Fig. 2 is an isometric view of the apparatus of the invention illustrating the action of the developing powder cloud during operation;

Fig. 3 is an isometric view of the agitator and impeller of the invention;

Fig. 4 is an isometric view of a xerographic plate supporting device that may be employed with the invention; and

Figs. 5, 6, 7 and 8 are isometric views of the invention embodying variously formed powder cloud chambers.

In the form of the invention shown in Fig. 1, the developing apparatus comprises a powder impelling and charg- 4 l ing mechanism mounted on a base 1 that also serves to support a container or cloud chamber 2, in which a cloud of xerographic developing powder is generated, and on which is mounted a plate holding device 3 for supporting a xerographic plate during the developing operation. The base member may conveniently be an adaptation of the type disclosed in Poplawski Patent No. 2,304,476, issued December 8, 1942, wherein a high-speed electric motor 5 (see also Fig. 3) is mounted in a base casting 6 so that its armature rotates about a vertical axis. A vertically extending shaft 7 is fixed to the armature of motor 5, for rotation therewith, and extends upwardly through the base casting 6 and through a suitable dust-tight seal 8 formed in a base plate 9 that is secured on base casting 6 and forms the bottom of the cloud chamber. Fixed on the lower portion of shaft 7 are a pair of developing powder agitator blades 10 that extend transversely of the axis of shaft 7 and at right angles to each other, and are formed to provide four separate cutting or agitating edges that are adapted to pass through a supply of xerographic developing powder placed on base plate 9 and to impel the powder upwardly.

Fixed on the upper end of shaft 7 is an impeller or rotor element 11 of a conventional centrifugal fan that includes a cylindrical stator element 12 having a spider element 13 spanning its upper periphery and a plurality of vertical standards 14 connected to its lowermost edge. Spider 13 is provided with a suitable bearing 15 in which the upper end of shaft 7 is journalled, and standards 14 are connected at their lower ends to a cylindrical element 16 that may be threaded into, or otherwise secured to, a circular collar 17 formed on base plate 9.

By this arrangement, the several elements of base member 1 form a unitary structure whereby particles of xerographic developing powder are separated from a supply placed on base plate 9 by the action of agitator blades 10 and are thrown directly into the centrifugal fan wherein they are further separated and accelerated in a vertical direction by impeller 11 and, at the same time, have a swirling movement imparted thereto. The stream of developing powder ejected by impeller 11 passes through three lobe-like openings in stator 12 formed by spider 13 whereby the powder stream is divided into three separate and distinct powder jets, each swirling individually under the influence of impeller 11. Impeller 11 and stator 12 of the centrifugal fan are preferably formed of stainless steel or cold rolled steel which is effective, upon highspeed frictional contact with developing powders of the type employed in xeroradiography, to impart a negative charge to the powder particles. Thus, in addition to impelling the powder particles upwardly, impeller 11 also serves to impart a homogeneous electrostatic charge to such particles whereby an extremely dense and negatively charged powder cloud is ejected upwardly from the base 7 member 1.

The cloud chamber or container 2 into which the powder cloud is forced includes a cylindrical member 20 (see Figs. 1, 2 and 3), preferably formed of sheet metallic material, having an annular flange formed on the lower edge thereof that seats on the upper surface of collar 17 of base member 1. Suitable seals are provided between cylinder 20 and collar 17 to form a completely dust-tight structure. The upper edge of cylinder 20 terminates at a point slightly below the upper surface of stator 12 and is provided with a square cap member 21 having a side dimension substantially equal to the diameter of cylinder 20. Extending upwardly from cap member 21 is a closed container in the form of an inverted pyramidal frustum which is preferably formed of sheet metal or other smooth surfaced material to permit powder particles to slide downwardly in the chamber as they fall out of the powder cloud formed therein. The side walls of cloud chamber 22 terminate in a substantially horizontal plane and are enclosed across their tops by a cover member 23 to form a dust-tight enclosure therewith.

xerographic plate to the powder cloud of developing material, a rectangular cut-out is formed substantially in the center of cover member 23 and a xerographic plate holder 3 is placed thereover. Plate holder 3 may be any convenient form of apparatus adapted to support a xerographic casette and permit the removal of the casette dark slide Without exposing the xerographic plate to exterior radiation. For example, the spring-urged, toggle actuated mechanism disclosed in above-mentioned copending application S. N. 436,482 is particularly adapted for this purpose and may readily be employed with the present apparatus. However, for simplicity of illustration in the present instance, the plate holding apparatus may comprise a simple rectangular frame (see also Fig. 4) having a rabbeted groove 26 formed on the inner surface of three sides thereof to accommodate the xerographic casette. Suitable leaf springs 27 are mounted on the top of frame 25 and serve to press the casette against the lower surface of groove 26 to form a substantially dust-tight seal therewith. Preferably, frame 25 is so formed that the lower edge of groove 26 holds the xerographic plate a slight distance above the under surface of cover 23 so as to minimize the possibility of directional effect in the developed radiograph.

To prepare the apparatus for operation, a charge of xerographic developing powder, usually two to three tablespoonfuls of a material such as stearated calcium carbonate having a grain size of .1 to 5.0 microns, is poured into cloud chamber 22 and permitted to settle to base plate 9. A xerographic casette with dark slide in place is then positioned under springs 27 of plate holder 3. The dark slide is then removed to expose the electrostatic latent image on the xerographic plate to the interior of the cloud chamber. Motor 5 is then started, preferably under control of a conventional timer switch to initiate the formation of the powder cloud. Agitator blades 19 then cut or chop into the pile of developing powder on base plate 9 at very high speed to propel the powder upwardly into impeller 11 of the centrifugal fan. The action of impeller 11 further separates the powder particles and concomitantly places a negative electrostatic charge on each and then propels them upwardly at an accelerated speed.

The swirling action of the individual powder jets tends to result in a similar swirling action of the entire powder cloud. However, the rectangular cross section of cloud chamber 32 results in the formation of eddy currents in the corners of the cloud chamber that are efiective to break up the swirling action of the main powder stream and to cause the formation of a very turbulent, non-directional powder cloud in the upper portion of the cloud chamber, the turbulent action being preferred to laminar flow in order to avoid directional effects during development. This dense cloud of charged powder particles is then literally pushed upwardly against the exposed surface of the xerographic plate by the action of the powder cloud stream being ejected by impeller 11. This powder cloud action is continued for a time period sutficient to effect complete development of the electrostatic latent image on the xerographic plate. The time period required is usually on the order of from five to ten seconds. When development is complete, the xerographic plate is re moved from plate holder 3 and the powder cloud is allowed to settle in the cloud chamber.

During development, agglomerates of powder particle material and spent powder particles tend to accumulate in the corners of cloud chamber 22 and tend to slide downwardly in the chamber rather rapidly because of the combination of gravitational forces and the minute vibration of the cloud chamber caused by the rotation of motor 5. As these particles approach base plate 9, their speed is accelerated by the action of the air stream that flows between container 20 and stator 12 of the centrifugal fan and then upwardly through impeller 11. Ideally, the spacing between stator 12 and container 20 is such to develop maximum air stream speed at this point. However, it is found that a spacing of approximately one-half inch between these elements produces practically acceptable results. By this arrangement, the particles that form the charged powder cloud remain in substantially continuous movement once they have been lifted from base plate g by agitators 10 or until they are deposited on a charged area of the xerographic plate. As the mixed powder and air stream continues to pass through impeller 11, any powder particle agglomerates that may have formed are broken up by the action of the impeller and all powder particles are again negatively charged by frictional contact with impeller 11 or stator 12. In addition to returning the powder to a position to be reactivated by the impeller, the high-speed air stream also is effective to eliminate tunneling effects in any portion of the powder supply remaining on base plate 9 so that, effectively, the entire powder charge placed in the container is in motion during the developing operation so that a dense powder cloud is maintained throughout the cloud chamber at all times.

Thus, there is disclosed an improved apparatus that is particularly adapted to the development of xeroradiographs wherein a very dense, highly charged powder cloud is generated and forced into surface contact with a xerographic plate to effect development thereof, and in which the use of air and developing powder material is completely regenerative except for the powder particles actually deposited on a plate.

Obviously, minor modifications may be prove the structure shown in the drawings from a practical standpoint. For example, centrifugal fan stator 12 may be inverted with reference to its present position and thereby provide a single opening through which a single jet of powder is ejected upwardly instead of the three jets, described above, whereby the swirling action of the powder may be made turbulent more conveniently. However, the present construction provides an arrangement whereby the upper bearin of shaft 7 may more conveniently be shielded from the powder. In addition, the horizontal shelf-like surfaces formed at the junction of cap 21 and cylinder 20 may preferably be filled in with a plastic or similar material to provide a smooth surface to facilitate the return of the powder to the air stream.

In the form of the invention shown in Figs. 5 to 8, inclusive, the principle of operation is identical with that described above. In each instance the base portion includes a high-speed motor for rotating coacting agitator blades 10 and a centrifugal fan including an impeller 11 and a stator 12, and the plate holder 3 comprises a device such as that disclosed in Fig. 4. The only d fferences in these devices is in the conformation of the cloud chamber, in each instance, whereby comparable efiects may be achieved.

In Fig. 5 the cloud chamber includes a conical frustum, preferably formed of sheet metal, that is joined to a cylindrical section 2%) such as that shown in Fig. 3. In this conformation, the powder cloud is generated as before and the swirling motion thereof is broken up by a plurality of bafiies 36, preferably either three or four, that are secured on the inner surface of frustum 35.

In the form of invention shown in Figs. 6 and 7 the cloud chamber is formed of a substantially vertical back wall 40 and sloping side and front walls whereby to form a closed container of varying rectangular cross-section, and wherein the cloud chamber is essentially divided into two portions by a vertical bafiie 44 extended between side walls 41. This construction is preferred for use in connection with developing powders which the I tend to cake or form agglomerates under certain operating conditions. In this arrangement, the powder cloud generated by impeller 11 is forced upwardly in rearward compartment 45 and over the top of bafile 44 and into forward compartment 46, wherein it is forced against the exposed surface of a xerographic plate held in plate holder 3.

made to im- Throughout the path of movement of the powder particles the varying cross-section of both compartments completely breaks up the uniform swirling action imparted by impeller 11 so that an extremely dense and nondirectional turbulent powder cloud is formed. In addition, baffle 44, rear wall 4t side walls 45 and cover 23 are elfective to slow down the upward movement of any powder particle agglomcrates and cause them to be returned to the lower portion of compartment 45. Powder particles that travel over the top of barrier 44, after their velocity is spent, drop downwardly in compartment 46 and are returned to the fan and agitator system by passing under the lower end of baffle 44.

The form of cloud chamber employed in Fig. 8 combines the advantages of those disclosed in Figs. 1 and 5. It comprises a section 47, in the form of an inverted frustum, that is mounted on a cylindrical section 26, of the type shown in Figs. 1 and 5, and includes four substantially triangular shaped portions 43 that are faired into each other by conic sections 49 whereby there is provided a transitional section that varies in cross-section from a circle at its line of contact with section 20 to a rectangle at its line of contact with an inverted pyramidal frustum section 59 mounted thereon. By this arrangement, the powder cloud generated by impeller 11 is permitted to swirl out of section 20, and then is made increasingly turbulent as it moves upwardly through the varying combinations of curved and flat surfaces formed by elements 48 and 49 of section 47, and through the constantly varying cross-sectional areas formed by the outwardly divergent side walls of section 50. This arrangement, although more complex from the standpoint of manufacture, is considered highly advantageous from the standpoint of decreased developing time and increased uniformity of development.

Although the invention, in each of its several forms, is described as applied to the development of xeroradiographs, it will be apparent to those skilled in this or related arts that it may readily be applied to any system in which it is required to form an extremely dense cloud of charged powder particles and to apply such cloud to a work surface. Therefore, since many changes could be made in the above constructions and many apparently widely different embodiments of diis invention could be made without departing from the scope thereof, it is intended that all matter contained in the specification and drawings be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. A xerographic developing apparatus comprising a developing powder agitator and a developing powder impeller arranged for rotation about a common axis, motive means for rotating said agitator and said impeller, a powder cloud chamber enclosing said agitator and said impeller, said chamber having upwardly divergent side walls, and a xerographic plate supporting means mounted on said chamber.

2. A xerographic developing apparatus comprising a developing powder agitator and a developing powder impeller arranged for rotation about a vertical axis, motive means for rotating said agitator and said impeller, a powder cloud chamber having upwardly divergent side walls above said powder agitating and impelling means and a cylindrical side wall extending downwardly therefrom and enclosing said agitator and said impeller, and a xerographic plate supporting means mounted on said chamber.

3. A xerographic developing apparatus comprising a developing powder agitator and a developing powder impeller arranged thereabove, said agitator and said impeller being mounted on a common shaft for rotation about a vertical axis, motive means for rotating said agitator and said impeller, a powder cloud chamber enclosing said agitator and said impeller, said chamber having upwardly divergent side walls terminating in a substantially horizontal plane, a cover member mounted on said side walls, and a xerographic plate supporting means mounted on said cover.

4. A xerographic developing apparatus comprising a support for a supply of xerographic developing powder, a vertical shaft extending through said support, powder agitator means fixed on said shaft above said support, powder impeller means fixed on said shaft above said agitator means, motive means for rotating said shaft, and a powder cloud chamber extending upwardly and out wardly from said support.

5. A xerographic developing apparatus comprising a support for a supply of xerographic developing powder, a vertical shaft extending through said support, powder agitator means fixed on said shaft above said support, powder impeller means fixed on said shaft above said agitator means, motive means for rotating said shaft, and a powder cloud chamber enclosing said agitator and impeller means and having side walls of upwardly and outwardly extending conformation mounted on said support.

6. A xerographic developing apparatus comprising a support for a supply of xerographic developing powder, a vertical shaft extending through said support, powder agitator means fixed on said shaft above said support, powder impeller means fixed on said shaft above said agitator means, motive means for rotating said shaft, and a powder cloud chamber extending upwardly and outwardly from said support, said powder cloud chamber including a cylindrical section enclosing said agitator and impeller means, and a section in the form of an inverted conical frustrum mounted on said cylindrical section.

7. A xerographic developing apparatus comprising a support for a supply of xerographic developing powder, a vertical shaft extending through said support, powder agitator means fixed on said shaft above said support, powder impeller means fixed on said shaft above said agitator means, motive means for rotating said shaft, and a powder cloud chamber extending upwardly and outwardly from said support, said powder cloud chamber including a cylindrical section enclosing said agitator and impeller means, and a section in the form of an inverted pyramidal frustum mounted on said cylindrical section.

8. A xerographic developing apparatus comprising a support for a supply of xerographic developing powder, a vertical shaft extending through said support, powder agitator means fixed on said shaft above said support, powder impeller means fixed on said shaft above said agitator means, motive means for rotating said shaft, and a powder cloud chamber extending upwardly and outwardly from said support, said powder cloud chamber including a cylindrical section enclosing said agitator and impeller means, and a section in the form of an inverted frustum mounted on said cylinder, said inverted frustum being of circular horizontal cross-section along its line of contact with said cylindrical section and of rectangular horizontal cross-section at its top and of continuously varying transitional cross-section therebetween.

9. A xerographic developing apparatus comprising a support for a supply of xerographic developing powder, a vertical shaft extending through said support, powder agitator means fixed on said shaft above said support, powder impeller means fixed on said shaft above said agitator means, motive means for rotating said shaft, and a powder cloud chamber extending upwardly and outwardly from said support, said powder cloud chamber including a substantially vertical side wall and three outwardly divergent side walls to form a chamber of continuously increasing rectangular cross-section.

10. A xerographic developing apparatus comprising a support for a supply of xerographic developing powder,

and a powder cloud chamber extending upwardly and outwardly from said support, said powder cloud chamber including a substantially vertical side wall and three outwardly divergent side walls to form a chamber of continuously increasing rectangular cross-section and terminating in a substantially horizontal plane, a cover member mounted on said side walls, and a xerographic plate holder mounted on said cover member in a position off-set from the axis of said vertical shaft.

11. A xerographic developing apparatus comprising a support for a supply of xerographic developing powder, a vertical shaft extending through said support, powder agitator means fixed on said shaft above said support, powder impeller means fixed on said shaft above said agitator means, motive means for rotating said shaft, and a powder cloud chamber extending upwardly and outwardly from said support, said powder cloud chamber including a substantially vertical side wall and three outwardly divergent side walls to form a chamber of continuously increasing rectangular cross-section terminating in a substantially horizontal plane, a cover member mounted on said side walls, a xerographic plate holder mounted on said cover in a position off-set from the axis of said vertical shaft, and a vertical baflie within said chamber partially separating the portion of said 10 chamber under said plate holder from the portion of said chamber over said powder impeller.

12. A xerographic developing apparatus comprising a support for a supply of xerographic developing powder, a vertical shaft extending through said support, powder agitator means fixed on said shaft above said support, powder impeller means fixed on said shaft above said agitator means, motive means for rotating said shaft, and a powder cloud chamber extending upwardly and outwardly from said support, said powder cloud chamber inciuding a cylindrical section enclosing said agitator and impeller means, and a section in the form of an inverted frustum mounted on said cylinder, said inverted frustum being of circular horizontal cross-section along its line of contact with said cylindrical section and of rectangular horizontal cross-section at its top and having a portion thereof of continuously varying transitional cross-section therebetween.

References Cited in the file of this patent UNITED STATES PATENTS 2,123,537 Marr July 12, 1938 2,221,776 Carlson Nov. 19, 1940 2,633,824 Dunn et al. Apr. 7, 1953 2,643,188 Meyer June 23, 1953 

1. A XEROGRAPHIC DEVELOPING APPARATUS COMPRISING A DEVELOPING POWDER AGITATOR AND A DEVELOPING POWDER IMPELLER ARRANGED FOR ROTATION ABOUT A COMMON AXIS, MOTIVE MEANS FOR ROTATING SAID AGITATOR AND SAID IMPELLER, A POWDER CLOUD CHAMBER ENCLOSING SAID AGITATOR AND SAID IMPELLER, SAID CHAMBER HAVING UPWARDLY DIVERGENT SIDE WALLS, AND A XEROGRAPHIC PLATE SUPPORTING MEANS MOUNTED ON SAID CHAMBER. 